1. Field of Endeavor
The present invention relates to the field of power plant technology. It refers to a method for operating a combined-cycle power plant and to a combined-cycle power plant useful for carrying out the method.
2. Brief Description of the Related Art
In order to increase the efficiency of combined-cycle power plants, the gas turbines in the prior art were run with high compression ratios. In the case of known gas turbines, such as ALSTOM's type GT26, compressed air is tapped from the compressor of the gas turbine in order to cool the combustor and parts of the turbine. The high temperatures of the tapped air which result during compression require cooling before the pressurized air (compressed air) can be used as cooling air. The compressed air can be cooled down, for example, in one or more once-through cooler(s) (OTC). In a combined-cycle power plant, water is tapped from the heat recovery steam generator for this purpose and delivered to the once-through cooler in order to cool the compressed air there. During this process, steam is produced in the once-through cooler. The superheated steam from the once-through cooler is fed back to the heat recovery steam generator.
It would now be desirable, for increasing the availability and flexibility of the operation, to configure and to operate a combined-cycle power plant so that it can be operated both in an operating mode which operates with two cycles (combined cycle, CC) and in an operating mode which operates with only one cycle (single cycle, SC).
If a combined-cycle power plant is operated with only one cycle, that is to say the gas turbine cycle, in which the heat recovery steam generator is not available, and if the power plant is changed over from the one operating mode to the other, the following problems result:                How is the once-through cooler, especially its pressure and temperature, to be controlled if it is switched?        In the case of the operating mode with only one cycle, where is the water for the once-through cooler to be found?        What is to happen to the steam which is produced in the once-through cooler?        
Although the operating mode with two cycles, from the point of view of efficiency, is of more interest to the operator than the operating mode with only one cycle, the latter can gain importance if the operator would like to have more flexibility during operation of the plant and improved availability without the entire plant having to be shut down. The operating mode with only one cycle can also be useful if a power plant is first constructed with only one cycle which later is expanded into a combined-cycle power plant.
Furthermore, a changeover from the operating mode with only one cycle to the operating mode with two cycles, as a fast start concept, is of importance for the power plant.